Issue 2 "To:" fields in email header - After upgrade to 18.0.37

[fanke]

I got the issue, that there are 2 similar "To:" fields in my Email headers in outgoing mail. So the mails are marked as spam or are unable to deliver. Does anyone know anything about that behavior?

To: mail@example.com
To: mail@example.com
From: "mail@example.com2"
<mail@example.com2>
Subject: asdf

 

[fanke]

Addition: This happens only for mails, coming from external mail client like thunderbird etc. Sending mails through the webmail service works and hast only 1 "To:" field.